1. Field of the Invention
The subject invention generally relates to turbomachines, and more particularly to an arrangement for providing an assembly for the mechanical attachment of a ceramic turbine blade to a metallic rotor disc.
2. Description of the Prior Art
The high pressure turbine of the subject invention is intended for use in gas turbine engines, and heretofore, turbines were manufactured with a high degree of precision and accuracy in order to achieve optimum performance and life. To that end, it was required to utilize precision machinery to achieve precision machining of the various parts of the turbine, such that the resulting assembly was characterized by close fitting tolerances. As is readily apparent, such manufacturing techniques are costly and time-consuming. Thus, it is desirable to obtain the manufacture of a gas turbine having wide tolerance components, yet the final assembly may be rapidly and readily assembled with fine precision and accuracy.
Another aspect of high performance turbines is to maximize the thermal efficiency and power output of the turbine. It is known that the thermal efficiency and power output of a turbine depends upon the temperature of the operating fluid. Higher thermal efficiency of a turbine is achieved when higher operating fluid temperatures are handled by the gas turbine. However, the main limiting factor in raising the temperature efficiency and power output is the physical capacity of the rotating blades. In general, turbine blades made from high-temperature resistant superalloys are capable of withstanding temperatures of approximately 1,800.degree.-2,000.degree. F. Advances in ceramics, such as silicon nitride (Si.sub.3 N.sub.4) and silicon carbide (SiC), will allow initial turbine temperatures in the range of 2,300.degree.-2,600.degree. F. Ceramics, however, are not as compliant as metals. Generally, a ceramic blade root with its inherent high notch sensitivity, low ductility, and low coefficient of thermal expansion is particularly prone to failure in the environment of a gas turbine. Generally, any introduction of tensile stresses in critical areas may propagate cracks in the ceramics, and the blade will fail. In other words, high point-loading and resulting stress concentration are of primary concern in a ceramic blade root attachment. Localized high load areas are due, in part, to uncontrollable variations in coefficients of friction, alignment, manufacturing tolerances, and tangential load. Therefore, in the design of the ceramic blade and rotor disc attachment, it is of primary importance to minimize tensile stresses when adapting the ceramic blades to the metallic rotor.
Heretofore, it has been known to cushion the area of contact between the ceramic blade root and the slot in the rotor by the use of an intermediate layer of woven or felted metal, by ceramic fiber, or asbestos cloth. Generally, this type of sandwich wadding is undesirable because it does not provide uniform support to the blade root, and possibly gives rise to slippage loss of material, stress overloading, matting, and pulverization of the intermediate materials. In addition, such slippage could result in upstream or downstream movement of the ceramic blade relative to the rotor disc.